The effect of porcupines
on the reduction in velocity of flow near the banks and their ability to induce
sedimentation has been investigated experimentally by various fraternities. An attempt
has been made to access the pattern of deposition caused by various configurations
of Porcupine field, the alignment of porcupine units, relationship between the
Manning’s roughness coefficient (‘n’ value) and the quantum of porcupine units,
etc.in an two dimensional Flume model at CWPRS, Pune for the present study. The
main objective of this study is to propose a suitable methodology in terms of
the placement, quantum, and orientation of porcupine units to increase the
resistance in the erosion prone reaches of a channel. The basic principle of
porcupine placing near the river bank is to offer resistance to flow thereby
reducing velocity and inducing sedimentation near the erosion prone banks. This
will in turn build the river banks resulting in shifting of channel course away
from the banks. The main advantage of RCC porcupine units are its flexibility
and also will be always in defense, if the river banks are attacked by the
channel migration in future.

Various
attempts have been made to overcome excessive erosion by constructing river
training works. Porcupine systems are one of the novel techniques which have
been adopted as a cost-effective method of river training. The effect of
porcupines on the velocity of flow and their ability to capture sediment has
been investigated experimentally. Also, an attempt has been made to logically
study the pattern of deposition caused by various configurations of Porcupine
field and hence to propose a preliminary design methodology.

Rivers
in alluvial plans are highly variable in their behavior and it is often
unpredictable. A stream, which is quite trouble free during low flow, may
attain a threatening condition during high stages. It may develop unforeseen
meander, break through embankment, attack town, and important structures,
outflank bridges and in general may create havoc. Therefore, whenever any
hydraulic structure is built across an alluvial stream, adequate measures in
form ofriver-training works must be
taken to establish the river course along a cretin alignment with a
predetermined cross-section, all there works which are constructed to train the
river are known as river-training works.

River
training’ refers to the structural measures which are taken to improve a river
and its banks. River training is an important component in the prevention and
mitigation of floods and general flood control, as well as in other activities
such as ensuring safe passage of a flood under hydraulic structure. For flash
flood mitigation, the main aim is to control the water discharge regime in the
watercourse by limiting its dynamic energy, thereby controlling the
morphological evolution of the watercourse (Colombo et al. 2002). River
training measures also manages sediment transportation and thus minimize bed
and bank erosion. Many river training structures are implemented in combination
with bioengineering techniques to lessen the negative effects on environment
and landscape. There are a number of types of river training structure. The selection
and design of the most appropriate structure depends largely on the site
conditions.

River
training is necessary in those reaches of the river where the river encounters
excessive erosion of bed or banks. Braiding, meandering, breaching of embankments,
damages of hydraulic structures, roads and railways, etc. are some of the
consequences of an untrained river. Various river training measures which are
commonly deployed include spurs/groynes (permeable and impermeable), submerged
vanes, bank pitching, guide vanes, bundling etc. Details about planning,
layout, design, and maintenance of permeable and impermeable type spurs are
covered in IRC: 89 (1997) & IS: 8408(1994). Porcupine Systems have also
been installed in big rivers in India like Brahmaputra and Ganga and have
yielded fairly good results. After Repeated failures of earthen spurs upstream
and downstream of the Farakka Barrage on the Ganga River (India), Central Water
Commission (CWC) Used RCCTo
protect the erosion of left bank. It is reported that the porcupines were very
effective and helped in siltation of the bank. Aamir and Sharma (2015) have
developed a rational design methodology for riverbank protection using RCC
Porcupines.

RCC
Porcupines have also been deployed at Majuli Island, Assam, India and they have
been found quite effective in reducing the intensity of the river Brahmaputra.
A Porcupine is a unit of the system which comprises six members of RCC which
are jointed together with the help of or on nuts and bolts to form a
tetrahedral frame. Each member is 2-4 m in length, depending upon the
requirements. At the time of concreting of members, holes are kept in the RCC
poles for the bolts. Generally, RCC poles of 3 m length are used having a cross
section of 15 cm × 15 cm. reinforcement is given using 4 Nos. of MS bars of 6
mm diameter, with stirrups at 15cm c/c. larger porcupines may also be used with
greater cross section and heavier reinforcement as per the requirement. Bolts
are normally 12-15 mm dimeter. Check nuts are to be provided for better grip.
Washers are required at both ends for better grip with the RCC members. RCC
porcupines should be connected together by wire rope and properly placed on the
ground to avoid any disturbance caused by the intensity of flow. Figure 1 shows
a three dimensional sketch of a typical RCC Porcupine unit.

Porcupines are a form of permeable
structure designed to reduce flow and trap sediment. They have pole-like
projections in all directions, resembling a porcupine with its quills sticking
into the air. They are used as flood control structures, and for river bank and
bed protection. Porcupines can be used in a line forming a spur into a river,
as silting aprons for larger spurs, and in a longitudinal line along an embankment.
Originally such devices were made of timber or bamboo, but these have a limited
lifespan. The use of wooden and bamboo porcupines combined with vegetation to
form a green wall is described in the chapter on bioengineering. This section
describes porcupines made of concrete.

There are two kinds of concrete
porcupine in common use: reinforced cement and pre-stressed cement (PSC).
Quality control of RCC struts is difficult because each strut cannot be tested
separately, although a rebound hammer can be used to test the uniformity of
strength throughout. PSC porcupines are better in terms of size, shape,
strength, concrete mix, and steel used.

Porcupines can be constructed in two
shapes, tetrahedral and prismatic. The following are their main uses.

Porcupine design

Tetrahedral
porcupines

The most common shape is
tetrahedral. The porcupine is formed by assembling six concrete struts of the
same length in a tetrahedral pattern. Individual struts are bolted together,
projecting beyond the joint. The bolts are passed through holes made at the
appropriate point using cheap polythene tubes during casting. The size of a
porcupine is denoted by the length of the individual struts, for example a 2 m
porcupine or 3 m porcupine. The struts of 2–3 m porcupines have a cross-section
of 10 x 10 cm. The individual struts are at 60 degrees to each other, thus a 2
m porcupine is about 1.7 m high and a 3 m porcupine 2.6 m high. The most common
sizes in use are 2, 2.5, and 3 m. Mounted or long boom cranes are necessary to
handle anything larger.

Fig. 1 Tetrahedral Porcupine

Prismatic porcupines

Prismatic porcupines are made
with nine concrete struts joined in the form of a prism. Two end triangles are
ford first and then joined together with three struts placed at the vertices.
Struts are bolted together as for tetrahedrons.

Fig. 2 Prismatic Porcupine

II.AIM:

Assign variation in
manning sand n value (Roughness coefficient) with the end of porcupine
system.

To study the flow
condition with the different setup of porcupine system with orientation.

To assize the
effectiveness of RCC porcupines with the different orientation and density
of porcupine.

IV.LITERATURE SURVEY

The various papers referred for this
area -Mohammad Aamir and Nayan
Sharma (11 March 2015) -presented
paper on River band protection with Porcupine systems development of rational
design methodology.On
this paper, the effect of porcupines on the velocity of flow and their ability
to capture sediment has been investigated experimentally. Also, an attempt has
been made to logically study the pattern of deposition caused by various
configurations of Porcupine field layout and hence to propose a preliminary
design methodology. Result show that there is a considerable reduction in the
flow velocity resulting in the deposition of sediment with porcupines offering
resistance to flow.

Presence
of porcupines can cause considerable reduction in the flow velocity, which
enhance further with increase in the number of units pleased in a series. A
second tier of porcupines can be pleased over the other to improve their
performance in the cases of high submergence. Densely configured porcupine field
can capture more sediment that a sparse field. Sediment capture capacity also
increases with increase in sediment concentration of flow. New suitable design
indices have been developed in this study which could be used to form the basis
for the preliminary design methodology and thus improve on the present day
conjectural approach. Preliminary design template is developed which can
provide the designer with the range of values of PFDI for different sediment
concentration and PFSI, to achieve the desired objective of erosion control,
moderate reclaim and heavy reclaim in the reach. Verification of the proposed
design with field applications is recommended.)

Ashok Kharya and Piyush Kumar
(10-14 April 2012) - presented
paper on RCC Porcupines an effective river bank protection measure-A case study of
protection of Majuli Island.On,
the paper gives an overview of the Permeable structures in the form of RCC
porcupines for bank protection measures and their successful application at
Majuli Island.

Siltation
measures in the form of R.C.C. porcupines have been found quite effective in
reducing intensity of erosion of the river Brahmaputra at Majuli Island, Assam.
The anti-erosion works for “Protection of Majuli Island” are still in progress.
However, from the observations made by the panel ofExperts in 2011, it can be Concluded that the
works in the form of RCC, porcupines which are cheap in cost, easy to
construct, sustainable and without significant adverse effects in upstream or
downstream or on opposite bank are a proven tool for checking erosion of river
banks.

V.METHODOLOGY

[1]Experimental
Setup:

A 16 m length, 1.2 m width, and
1.5 m depth flume would be used at CWPRS, Pune. A depth of 0.5 m thick Khaswa
sand having mean diameter of 0.32 mm (d50) would be placed in side the flume
with certain initial bed slope. A divider using GI sheet would be inserted
exactly at the centre of the flume from upstream of flume up to the flume from
upstream of flume up to the end of glass portion as shown in figure Fig. 3 channel section

A
2 feet standing wave flume would be used at up stream of flume for discharge
measurement. The porcupine unit would be fabricated using 8 mm GI wire
(photo…). Odd type propeller current mater, depth rod, digital pointer gauge
would be used for extracting different hydraulic parameters.

[2]Experimental
procedure:

Initially
a discharge (based on the flume set up/ capacity of the flume) would be fed
into the flume after measuring it through the 2’ standing wave flume. A
constant depth of flow would be maintained in the flume. The flume would be run
for about 1.5 to 2 hours until the mobile bed of the flume is stabilized and
attain equilibrium. There after, the velocity, depth, bed slope etc. would be
measured in the two different channels. Based on these parameters the manning’s
“n” value (roughness coefficient) and discharge in the two bifurcated channels
would be computed. The above experiments are for the existing or without
porcupine unit conditions.

Similar experiments are erred out
by placing a row of porcupine units at one of the bifurcated channels. All
other procedures/ measurements are done as explained above. Like wise the different
hydraulic parameters are extracted by providing additrmal rows of porcupines.
The comparisons shall be made in the parameters obtained from both the
bifurcated channel. This would establish a relationship of manning’s “n” value
u/s the number of porcupines rows. This would help in determining the quantum
of resistances to be offered in the channel to absorb the momentum of flow/
energy so that the adjust channel could be aestivated on higher degree. Apart
from these experiments other alterative such as placing the porcupine units in
a staggered way, changes in the shape of porcupine unit etc. could be during
the course of experiments.